We Want a Shrubbery

Janice_Kaspersen_Blog

We know trees are good for the urban environment, despite the occasional complaint that they shed too many leaves or eventually break up sidewalks. Now researchers in Europe are promoting what they say is an even better idea: hedges.

Prashant Kumar of Surrey University and others have just published research showing that low hedges trap pollutants from car exhaust—being, as they are, right down there at the level of the tailpipe—much better than the canopies of tall trees do. The study doesn’t recommend replacing trees with hedges, but rather planting both wherever possible. Planting hedges close to the road, where they will trap pollutants before they dissipate, is ideal.

In addition to trees’ other benefits—reducing the heat island effect, removing carbon dioxide from the air, and possibly increasing real estate prices for the properties near them—they also reduce peak flows of stormwater runoff. They hold a certain amount of water in the canopy for a time after it rains, and they also transpire quite a bit of water daily, depending on the type of tree. How good are hedges at doing the same?

Lots of data exists for trees’ performance. This article from the October 2016 issue of Stormwater,  “Give Me the Numbers: How trees and urban forest systems really affect stormwater runoff” by Aarin Teague and Eric Kuehler, offers some details. There are also plenty of estimates out there for intensive and extensive green roofs, and for various types of vegetated cover. For example, this (Table 3-5) and many other sources provide runoff curve numbers for different land uses, including general types of vegetation. What I haven’t been able to find are specific numbers for trees versus low-growing plants—boxwood, privet, oleander, and so on. If you’re aware of research in this area, please leave a comment below.

Just as trees have their critics, though, one complaint about hedges is that they require more frequent maintenance than trees do to keep them looking good. The UK researchers point out that in some areas of the country, cities are now charging more to collect green waste, which might also prompt people to rip out their hedges rather than trim them.

Kumar and his coauthors have published their results in the journal Atmospheric Environment. (Note that their use of the term green infrastructure here is different from how we typically use the term in discussions of stormwater.)SW_bug_web

Comments
  • Jonathan McClelland.

    Some advantages that trees have over shrubbery for roadside and median plantings are:
    1. less visual obstruction for potential hazards (pedestrians, stray animals, cross traffic).
    2. simpler irrigation systems, due to fewer emitters
    3. potential for increased carbon sequestration
    4. more effective control of the “heat island” effect of paved surfaces.
    I would think that leaf removal vs. shrub pruning would be fairly even labor trade-offs .
    I can think of some situations where hedges or other forms of shrubbery would be preferable to trees, but to suggest that they might be “the next silver bullet” for combating ICE pollution is ludicrous. How about just generating less pollution by making better choices in our fleet?
    On a related topic, I wonder why urban median plantings and sidewalk separators using raised curbs have been traditionally designed as berms that shed runoff instead as basins that are capable of collecting runoff from the road surface ? I know, the answer is cheaper construction costs, but with the increased regulations for mitigating stormwater runoff requiring dedicated collection acreage, it’s time to start including more sophisticated design into the “bean counter” process.

    Reply
  • Diane Minick.

    As a stormwater landscaper, I deal with this many times. Hedges are very dense and plants like privet are not native and are quite invasive. Native shrubs require less maintenance and are more open and give a softer look. Depending on the type, they can form hedge-like clusters. Itea Virginia for one, can be places in a depressed median that has stormwater from the road coming in. As to the transpiration of oxygen, trees are probably best at it due to their size and more strength in their vascular systems, but shrubs will inhale much CO2 and the O2 they exhale will e at the pedestrian level which would be a plus.

    Reply
  • Thomas E..

    We’re trying to get away from traditional hedges! The cost of maintaining them is too high due to both labor intensity and polluting mechanical equipment. Hopefully, this study will set a precedent for further research into space-appropriate understory plants and density requirements. But let’s try to get away from forced hedging practices that cause excessive labor and pollution.

    Reply
  • Kip S..

    Katchall Stormwater Filtration Systems, LLC (California) has been utilizing vegetated stormwater filtration devices for 15+ years and specializes in low-growth, no-visual impairment filtration vaults. Our experience has been (if properly designed), low-growth vegetation actually captures & utilizes more runoff than do smaller “tree-box” units. Maintenance isn’t an issue, if you’re worried about “polluting mechanical equipment” use a battery powered trimmer !!

    Reply

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